Fish farming is an industry growing at a rapid rate, which is driven by the demand for increased quantities of food resources and the decrease of natural sources of fish in lakes and oceans. Fish farming usually involves floating cages, which are positioned just offshore in fjords, seas, lakes or other natural bodies of water. The fish cages are usually wide cages comprising a rigid cylindrical frame structure positioned horizontally on the surface of the body of water and a net system allowing water to flow into and out of the cage. The net system is made with net meshes of sufficiently small size to retain the fish inside the cage. Such conventional cages typically have a circular bottom net, a cylindrical sidewall net extending to the water surface and an open top end, which may also be covered by a net.
In the most common setup for fish farming plants, a number of floating net cages are anchored close to shore using buoys and weights to stabilize the cages in the water. An optimal installation for fish farms of this sort is in sheltered waters such as bays and fjords and these setups often comprise support frames with gangways for operational purposes, such as maintenance and feeding.
Surface cages, however, are sensitive to severe weather conditions, such as high winds, waves and ice, which can be a serious threat in northern areas. Submersible cages have been suggested and tested. However, these types of cages have not become commercially feasible due to problems with stability and handling.
For example, PCT Publication No. WO 82/03152 to Rene describes a semi-submersible rotatable cage having two or more rings encircling a horizontal central axle. Wire rigging attaches each of the rings and a netting in place over top of the rigging to form a cylindrical cage structure. The cage is rotated while semi-submerged by the systemic inflation of a plurality of buoyancy elements, such as balloons. Since the cage relies on the structural integrity of the netting and the wire rigging to maintain the shape and structure of the cage, damage to either the netting or wire rigging can result in catastrophic structural failure to the cage.
French Patent Application No. 2420920 to Arin also describes a submersible rotatable cage having two or more parallel circular floats that may be connected by a plurality of bars, thereby forming a cylindrical frame. Water and/or air may be introduced into one or more quadrants of the circular floats in a controlled manner to affect the buoyancy of the cage and allow rotation of the cage when partially submerged. The filling of the circular floats with water and/or air using quadrants results in a sporadic rotation of the cage, which can cause undue stress to the inhabitants of the cage as well as the frame itself.
Accordingly, there is a need for a simple, robust, durable and commercially practical cage for cultivating marine animals, which is easy to manufacture and install and can be maintained underwater and readily manipulated to allow for the collection of animals, sorting of animals and maintenance of the cage.